Field of the Invention
This invention relates to a die stem for use in an extrusion press, and particularly to a sleeve type die stem.
Description of the Prior Art
In the die stem of a type adapted to be connected at its end to a die holder incorporating a die or a die backer, such as a die stem for use in an indirect extrusion press, a compression force is generated in the connecting construction during the extrusion so as to shorten the life of the die stem, thus hindering the safe operation of the whole apparatus.
As is well known, in the indirect extrusion press, it is a common practice to use a die stem unitarily connected at its end to a die holder, through a connecting construction such as a screw or a bayonet, to a die holder incorporating a die, die backer and so forth. In such a die stem, however, the stress concentration factor is impractically large. In fact, this factor is as large as on the order of 2 to 3 or larger. During the extrusion, the full extrusion power is concentrated on the die stem and the die holder, so that the resulting compression stress attains a considerably high level. Consequently, the life of the conventional die stem is rendered impractically short, so as to jeopardize the safe operation of the whole apparatus.
FIGS. 1 and 2 show typical examples of conventional die stems. More specifically, FIG. 1. shows a die stem of a type having no sleeve. In this die stem, as will be seen from the illustration thereof, the die stem 3 is held by a die stem slide 1 through a die stem retainer 2 or the like. In such illustrated example, a thread 3b is formed at the end portion of the internal bore 3a of the die stem 3. On the other hand, a thread 6b adapted to engage the thread 3b is formed on the outer periphery of a rear end projection 6a of a die holder 6 incorporating a die 4 and a die backer 5. In use, the die stem 3 and the die holder 6 are made unitary, through the screwing engagement of both threads 3a, 6b. In this state, the front end of the die stem 3 and the rear end of the die holder 6 are held in contact with each other, as shown in FIG. 1.
FIG. 2 shows another example of a conventional die stem, in which a sleeve is incorporated. In this type of the die stem, an inner sleever 7 is received by an inner bore 3a of the die stem 3 which is held by the die stem slide 1 through the medium of a die stem retainer 2 or the like. The sleeve 7 is fixed at its rear end to the rear end of the die stem 3 by means of a nut 8 or like means. The inner sleeve 7 is provided at its end with an outer peripheral thread 7a, while the rear end projection 6a of a die holder 6 incorporating a die 4 and a die backer 5 is provided with an inner peripheral thread 6b adapted for engagement with the thread 7a of the inner sleeve. In use, the die stem 3 and the die holder 6 are fixed unitarily to each other, through the medium of the inner sleeve 7, by screwing engagement. In this type of the die stem, the die stem 3 is made to contact at its front end surface the rear end surface of the rear end projection 6a of the die holder 6.
As the extrusion power is exerted as shown by the arrow F in the extrusion stroke of the extrusion press, a large nominal compression stress is generated at the thread 3b of the die stem 3 in case of the arrangement as shown in FIG. 1, and at the thread 6b of the die holder 6 in case of the arrangement as shown in FIG. 2. Further, due to the stress concentration at the bottom of the thread, a stress 2 to 3 times as large as the nominal compression stress is generated at the thread and the bottom of the teeth. This large stress often causes a local plastic deformation at the bottom of the screw, resulting in a shortened life of the die stem. Alternatively, in order to avoid such an inconvenience through reducing the level of the nominal compression stress, it is necessary to reduce the inner diameter of the die holder 6. Although the connecting constructions in these examples are screws, the same disadvantages are caused in the die stem having a connecting construction constituted by bayonets.